I want to spin up some VMs to use purely as run time machines. We do very minimal automation with desktop browsers. The vast majority of our automation deals with our native mobile apps (iOS and Android) and the RxBrowser. We have separate machines that runs our Simulators and Emulators. It goes without saying that the VMs would need to communicate with the devices/simulators via LAN or WiFi. I was wondering if people with real world experience can recommend the VM system specs and hard drive space.

We used Hyper-V sccessfully (on Windows 10 / 64 bit).
You could install the visual studio emulator for android on your local machine,
or off course a real device.
From your VM you are able to use the emulator or device by network.
Your machine has to be in the same wifi and ranorex on VM requires the floating license.

Ranorex works also fine with VMware, but in this case you can't use VS emulator, because this emulators run also as Hyper-V. A combination of Hyper-V and VMware runing on the same host is not possible.

The minimum hardware requirements needed to run Ranorex successfully are pretty low.
Ranorex does not require lot of system resources - which is good.

In our case we have set up th VM's to have sufficient memory to host the OS, SUT and Ranorex and still have at least 1GB not used memory.

If you know the typical resources of the customer target system, adjust VM resources to match them.
Generally said, it is more likely that test cases fail due to lower system resources than vice versa.